Normalizing is a type of heat treatment applicable to ferrous metals only. It
differs from annealing in that the metal is heated to a higher temperature and
then removed from the furnace for air cooling.

The purpose of normalizing is to remove the internal stresses induced by heat
treating, welding, casting, forg­ing, forming, or machining. Stress, if not
controlled, leads to metal failure; therefore, before hardening steel, you
should normalize it first to ensure the maximum desired results. Usually,
low-carbon steels do not re­quire normalizing; however, if these steels are
normal­ized, no harmful effects result. Castings are usually annealed, rather
than normalized; however, some cast­ings require the normalizing treatment.
Table 2-2 shows the approximate soaking periods for normalizing steel. Note that
the soaking time varies with the thickness of the metal.

Normalized steels are harder and stronger than an­nealed steels. In the
normalized condition, steel is much tougher than in any other structural
condition. Parts subjected to impact and those that require maximum toughness
with resistance to external stress are usually normalized. In normalizing, the
mass of metal has an influence on the cooling rate and on the resulting
struc­ture. Thin pieces cool faster and are harder after normal­izing than thick
ones. In annealing (furnace cooling), the hardness of the two are about the
same.

HARDENING

The hardening treatment for most steels consists of heating the steel to a
set temperature and then cooling it rapidly by plunging it into oil, water, or
brine. Most steels require rapid cooling (quenching) for hardening but a few can
be air-cooled with the same results. Hardening increases the hardness and
strength of the steel, but makes it less ductile. Generally, the harder the
steel, the more brittle it becomes. To remove some of the brittleness, you
should temper the steel after hard­ening.

Many nonferrous metals can be hardened and their strength increased by
controlled heating and rapid cool­ing. In this case, the process is called heat
treatment, rather than hardening.

To harden steel, you cool the metal rapidly after thoroughly soaking it at a
temperature slightly above its upper critical point. The approximate soaking
periods for hardening steel are listed in table 2-2. The addition of alloys to
steel decreases the cooling rate required to produce hardness. A decrease in the
cooling rate is an advantage, since it lessens the danger of cracking and
warping.

Pure iron, wrought iron, and extremely low-carbon steels have very little
hardening properties and are dif­ficult to harden by heat treatment. Cast iron
has limited capabilities for hardening. When you cool cast iron rapidly, it
forms white iron, which is hard and brittle. And when you cool it slowly, it
forms gray iron, which is soft but brittle under impact.

In plain carbon steel, the maximum hardness ob­tained by heat treatment
depends almost entirely on the carbon content of the steel. As the carbon
content in­creases, the hardening ability of the steel increases; however, this
capability of hardening with an increase in carbon content continues only to a
certain point. In practice, 0.80 percent carbon is required for maximum
hardness. When you increase the carbon content beyond 0.80 percent, there is no
increase in hardness, but there is an increase in wear resistance. This increase
in wear resistance is due to the formation of a substance called hard cementite.

When you alloy steel to increase its hardness, the alloys make the carbon
more effective in increasing hardness and strength. Because of this, the carbon
con­tent required to produce maximum hardness is lower than it is for plain
carbon steels. Usually, alloy steels are superior to carbon steels.

Carbon steels are usually quenched in brine or water, and alloy steels are
generally quenched in oil. When hardening carbon steel, remember that you must
cool the steel to below 1000°F in less than 1 second. When you add alloys to
steel, the time limit for the temperature to drop below 1000°F increases above
the 1-second limit, and a slower quenching medium can produce the desired
hardness.

Quenching produces extremely high internal stresses in steel, and to relieve
them, you can temper the steel just before it becomes cold. The part is removed
from the quenching bath at a temperature of about 200°F and allowed to air-cool.
The temperature range from 200°F down to room temperature is called the
"cracking range" and you do not want the steel to pass through it.

In the following paragraphs, we discuss the differ­ent methods of hardening
that are commercially used. In the Seabees, we use a rapid surface hardening
com­pound called "Case" that can be ordered through the Navy supply system.
Information on the use of "Case" is located in the Welding Materials Handbook,
P-433. Case Hardening

Case hardening produces a hard, wear-resistant sur­face or case over a
strong, tough core. The principal forms of casehardening are carburizing,
cyaniding, and nitriding. Only ferrous metals are case-hardened.

Case hardening is ideal for parts that require a wear-resistant surface and
must be tough enough inter­nally to withstand heavy loading. The steels best
suited for case hardening are the low-carbon and low-alloy series. When
high-carbon steels are case-hardened, the hardness penetrates the core and
causes brittleness. In case hardening, you change the surface of the metal
chemically by introducing a high carbide or nitride content. The core remains
chemically unaffected. When heat-treated, the high-carbon surface responds to
hard­ening, and the core toughens.

CARBURIZING.- Carburizing is a case-harden­ing process by which carbon is
added to the surface of low-carbon steel. This results in a carburized steel
that has a high-carbon surface and a low-carbon interior. When the carburized
steel is heat-treated, the case be­comes hardened and the core remains soft and
tough.

Two methods are used for carburizing steel. One method consists of heating
the steel in a furnace con­taining a carbon monoxide atmosphere. The other
method has the steel placed in a container packed with charcoal or some other
carbon-rich material and then heated in a furnace. To cool the parts, you can
leave the container in the furnace to cool or remove it and let it air cool. In
both cases, the parts become annealed during the slow cooling. The depth of the
carbon penetration depends on the length of the soaking period. With to­day's
methods, carburizing is almost exclusively done by gas atmospheres.

CYANIDING.- This process is a type of case hardening that is fast and
efficient. Preheated steel is dipped into a heated cyanide bath and allowed to
soak. Upon removal, it is quenched and then rinsed to remove any residual
cyanide. This process produces a thin, hard shell that is harder than the one
produced by carburizing and can be completed in 20 to 30 minutes vice several
hours. The major drawback is that cyanide salts are a deadly poison.

NITRIDING.- This case-hardening method pro­duces the hardest surface of any
of the hardening proc­esses. It differs from the other methods in that the
individual parts have been heat-treated and tempered before nitriding. The parts
are then heated in a furnace that has an ammonia gas atmosphere. No quenching is
required so there is no worry about warping or other types of distortion. This
process is used to case harden items, such as gears, cylinder sleeves, camshafts
and other engine parts, that need to be wear resistant and operate in high-heat
areas.

Flame Hardening

Flame hardening is another procedure that is used to harden the surface of
metal parts. When you use an oxyacetylene flame, a thin layer at the surface of
the part is rapidly heated to its critical temperature and then immediately
quenched by a combination of a water spray and the cold base metal. This process
produces a thin, hardened surface, and at the same time, the internal parts
retain their original properties. Whether the proc­ess is manual or mechanical,
a close watch must be maintained, since the torches heat the metal rapidly and
the temperatures are usually determined visually.

Flame hardening may be either manual or automat­ic. Automatic equipment
produces uniform results and is more desirable. Most automatic machines have
vari­able travel speeds and can be adapted to parts of various sizes and shapes.
The size and shape of the torch de­pends on the part. The torch consists of a
mixing head, straight extension tube, 90-degree extension head, an adjustable
yoke, and a water-cooled tip. Practically any shape or size flame-hardening tip
is available (fig. 2-1).

Figure 2-1.-Progressive hardening torch tip.

Tips are produced that can be used for hardening flats, rounds, gears, cams,
cylinders, and other regular or irregular shapes.

In hardening localized areas, you should heat the metal with a standard
hand-held welding torch. Adjust the torch flame to neutral (see chapter 4) for
normal heating; however, in corners and grooves, use a slightly oxidizing flame
to keep the torch from sputtering. You also should particularly guard against
overheating in comers and grooves. If dark streaks appear on the metal surface,
this is a sign of overheating, and you need to increase the distance between the
flame and the metal.

For the best heating results, hold the torch with the tip of the inner cone
about an eighth of an inch from the surface and direct the flame at right angles
to the metal. Sometimes it is necessary to change this angle to obtain better
results; however, you rarely find a deviation of more than 30 degrees. Regulate
the speed of torch travel according to the type of metal, the mass and shape of
the part, and the depth of hardness desired.

In addition, you must select the steel according to the properties desired.
Select carbon steel when surface hardness is the primary factor and alloy steel
when the physical properties of the core are also factors. Plain carbon steels
should contain more than 0.35% carbon for good results inflame hardening. For
water quench­ing, the effective carbon range is from 0.40% to 0.70%. Parts with
a carbon content of more than 0.70% are likely to surface crack unless the
heating and quenching rate are carefully controlled.

The surface hardness of a flame-hardened section is equal to a section that
was hardened by furnace heating and quenching. The decrease in hardness between
the case and the core is gradual. Since the core is not affected by flame
hardening, there is little danger of spalling or flaking while the part is in
use. Thus flame

Figure 2-2.-Progressive hardening.

hardening produces a hard case that is highly resistant to wear and a core
that retains its original properties.

Flame hardening can be divided into five general methods: stationary,
circular band progressive, straight­line progressive, spiral band progressive,
and circular band spinning.

STATIONARY METHOD.- In this method the torch and the metal part are both held
stationary.

CIRCULAR BAND PROGRESSIVE METHOD.­This method is used for hardening outside
surfaces of round sections. Usually, the object is rotated in front of a
stationary torch at a surface speed of from 3 to 12 inches per minute. The
heating and quenching are done progressively, as the part rotates; therefore,
when the part has completed one rotation, a hardened band encir­cles the part.
The width of the hardened band depends upon the width of the torch tip. To
harden the full length of a long section, you can move the torch and repeat the
process over and over until the part is completely hard­ened. Each pass or path
of the torch should overlap the previous one to prevent soft spots.

STRAIGHT-LINE PROGRESSIVE METHOD.­With the straight-line progressive method,
the torch travels along the surface, treating a strip that is about the same
width as the torch tip. To harden wider areas, you move the torch and repeat the
process. Figure 2-2 is an example of progressive hardening.

SPIRAL BAND PROGRESSIVE METHOD.­For this technique a cylindrical part is
mounted between lathe centers, and a torch with an adjustable holder is mounted
on the lathe carriage. As the part rotates, the torch moves parallel to the
surface of the part. This travel is synchronized with the parts rotary motion to
produce a continuous band of hardness. Heating and quenching occur at the same
time. The number of torches required depends on the diameter of the part, but
seldom are more than two torches used.

CIRCULAR BAND SPINNING METHOD.­The circular band spinning method provides the
best results for hardening cylindrical parts of small or me­dium diameters. The
part is mounted between lathe centers and turned at a high rate of speed pasta
station­ary torch. Enough torches are placed side by side to heat the entire
part. The part can be quenched by water flowing from the torch tips or in a
separate operation.

When you perform heating and quenching as sepa­rate operations, the tips are
water-cooled internally, but no water sprays onto the surface of the part.

In flame hardening, you should follow the same safety precautions that apply
to welding (see chapter 3). In particular, guard against holding the flame too
close to the surface and overheating the metal. In judging the temperature of
the metal, remember that the flame makes the metal appear colder than it
actually is.